Thickened aqueous fluids are widely used in industry. Such fluids are typically thickened in order to increase viscosity or suspend particles. The thickeners may also stabilize emulsions; flocculate particles; act as binders, film formers, lubricants and friction reducers; or perform many other functions. They are commonly used in paints and coatings, inks, textile finishing, agricultural chemicals, cosmetics, foods, and many other industries. One industry of particular interest is the energy industry, where aqueous thickened fluids are used in many well treatments such as drilling, completion, fracturing, acidizing, cleanout, gravel packing and the like.
Water-based well treatment fluids often contain a hydratable polymer that acts to thicken the fluid and may be further thickened by chemical crosslinking. Such a polymer typically is made available in either a powder for or a suspended form in a carrier fluid such as a hydrocarbon such as No. 2 diesel oil, and is hydrated upon the surface. (Such suspensions are often called slurries; the terms “slurry” and “suspension” are considered to be interchangeable, although the term suspension will be used here.) In general, the powder or suspension must provide polymers that hydrate rapidly. Dry polymer particles must first be dispersed so that individual particles can absorb water; otherwise, some of the polymer will not hydrate and lumps (that contain dry powder inside a gelatinous coating) will form. For this reason, suspensions are usually preferred over dry polymer particles, if stable hydratable suspensions can be prepared. Once the polymer is dispersed, its ability to absorb water will determine the hydration rate. Natural polymers are most commonly used, especially polysaccharides, such as guar and derivatives of guar such as hydroxypropyl guar (HPG), carboxymethylhydroxypropyl guar (CMHPG), carboxymethyl guar (CMG), and hydrophobically modified guar, or hydrophobically modified guar derivatives.
Fluids may be batch-mixed or continuously mixed. For batch-mixed, water-based fluids, additives such as bactericide, polymer, salt, buffer, acid or base, clay stabilizer, etc., are mixed together in tanks with the water before pumping. The polymer is given sufficient time to hydrate in the tanks before the job begins. Batch mixing affords the best opportunity for quality assurance. Unfortunately, it also results in wasted materials if not all can be used. Obstacles facing the well-treatment industry also include large costs and environmental effects of operating and conducting treatments. Large costs are associated with storing and maintaining numerous liquids in large quantities in various, and sometimes remote, regions of the world. Further, the environmental effects of spillage and relatively large leftover quantities of fluid on site (tank bottoms) are increasingly becoming a problem for operators, as disposal of fluids is particularly troublesome under newer and more stringent environmental regulations. From a cost standpoint, continuously mixed fluid is more desirable. In this mode, all materials are added on the fly, so there is no wasted fluid and no unnecessary expense. Concentrated suspensions of guar or guar derivatives or the like in diesel or other liquids (called carrier fluids) were developed so that the polymer could be accurately metered and so that it would disperse and hydrate rapidly enough for continuous mixing (Constien, V. G., et al., Oil & Gas Journal, 86, No. 23, pp. 49-54, 1988; Yeager, R. R, and Bailey, D. E., SPE 17535, 1988). Because of environmental considerations and disposal costs, most aqueous-based fracturing fluids are now continuously mixed. Of course, it would still be desirable if the carrier fluid were environmentally friendly, in case of accidental spills.
Concentrated suspensions useful in a continuous process for supplying a viscous fluid for treatment of subterranean formations have been developed. Such a concentrated suspension typically involves a polymer suspension wherein a hydratable polymer is dispersed in a hydrophobic solvent (generally an oil-based fluid) in combination with a suspension agent and a surfactant, with or without other optional additives commonly employed in well treatment applications. Because of the inherent dispersion of the hydratable polymer in the oil-based fluid (i.e., lack of affinity for each other), such concentrated suspensions tend to eliminate lumping and premature gelation problems and tend to optimize initial dispersion when added to water. However, the rate of hydration of the polymer is still a critical factor particularly in continuous mix applications wherein the necessary hydration and associated viscosity rise must take place over a relatively short time span corresponding to the residence time of the fluids during the continuous mix procedure. Also, surfactants are needed when hydrophobic solvents such as diesel are used because without them there is a delay in wetting the polymer particle and subsequently hydrating it when the suspension is mixed with water for blending the oilfield service fluid; the surfactant helps to remove the oil, such as diesel, from the particles and allows the polymer particles to hydrate. In such applications, hydration is the process by which the hydratable polymer absorbs water, and is necessary for the development of increased viscosity. Once the polymer is dispersed, its ability to absorb water will dictate hydration or hydration rate. Several factors will determine how readily the polymer will hydrate or develop viscosity; e.g., the pH of the system (particularly for natural polymers), the amount of mechanical shear applied in the initial mixing phase, the polymer particle size, the concentration of salts, and the polymer concentration. Hydration rate can be influenced through pH control agents, which may be blended with the polymer suspension or added to the aqueous medium.
Typical suspensions used in the industry might comprise about 45 to 55% of No. 2 diesel oil as a hydrophobic solvent and about 45 to 55% of a “dry” polysaccharide powder (if the suspension is to be used to make a viscous aqueous fluid). The “dry” polysaccharide powder typically contains about 1 to 3% water, up to about 1% of a surfactant or dispersant, up to about 1% of an agent such as silica, and about 1 to 3% of a clay. The silica keeps the polysaccharide free flowing when it is in the “dry” powder form, and therefore is often termed a “free-flow” agent or additive. The surface of the clay is typically treated with an organic material to render it hydrophobic; such clays are termed organophilic. These clays aid in maintaining the polysaccharide in suspension when the dry polysaccharide is subsequently added to the carrier fluid to form the suspension.
Many oil-based fluids have been used as hydrophobic solvents. Diesel is most common. As an alternative, U.S. Pat. No. 5,091,448 describes use of a mixture of an oil, especially an isoparaffin oil, and a resin such as hydrogenated styrene/isoprene block copolymers. Attempts to avoid water-immiscible hydrocarbons have been made, to reduce toxicity, improve biodegradability, and avoid sheens on water. U.S. Pat. No. 4,176,107 describes water-soluble polyalkylene glycols having at least four ethyleneoxy units each or at least three propyleneoxy units each; or water-soluble polyethoxylated alcohols, polyethoxylated alkyl phenols or polyethoxylated fatty acids all having at least three ethyleneoxy units each. These expensive materials may be diluted with less expensive co-solvents: for example alcohols such as 2-octanol, tri- and tetraethyleneglycols, ethers such as the methyl, ethyl, propyl or butyl ethers of glycols, ketones such as diacetone alcohol, amides such as dimethylformamide, and esters such as methoxyethyl acetate. U.S. Pat. No. 4,799,962 describes a mixture of water and low molecular weight (about 200 to about 700, preferably at least 250) and high molecular weight (greater than about 3000) polyethylene glycols or methoxylated polyethylene glycols. U.S. Pat. No. 5,969,012 describes polyalkylene glycols with amine phosphate ester salt stabilizers. These alternatives do not solve all the problems associated with the use of such concentrated suspensions. For example, they may be too viscous, particularly at lower temperatures.
Unfortunately, the surfactants that are needed add to the cost and complexity of the system, and very importantly, surfactants commonly used in these systems, such as nonyl phenol ethoxylates, are toxic to many biota. They are, for example, banned for use in the North Sea. The surfactant or dispersant used in suspensions prepared in the past is generally any such material that is capable of both wetting the suspension agent for dispersion in the hydrophobic solvent and stripping the hydrocarbon from the polymer particles upon introduction into the water to allow hydration of the polymer. As such, the required surfactants typically included ethoxylated nonylphenol, alkyloxylated alcohols having polymerized propylene oxide and/or ethylene oxide, other copolymers of propylene oxide and ethylene oxide or the like, glycol ethers, various derivatives thereof and mixtures. The surfactant is often an ethyoxylated nonyl phenol employed at a concentration of about 0.2 to about 5.0% by weight of the concentrate. These surfactants typically are not environmentally friendly. There is a need in many industries, such as the oilfield industry, for compositions and methods of making environmentally friendly, surfactant-free suspensions of water-soluble materials in non-aqueous solutions and then for using the suspensions to make aqueous fluids.